In recent years, attention has been paid to fuel cells as clean energy sources. The use purposes thereof are mainly power generation for home use, power generation for business, power generation for automobiles, and others, and researches for improving the cells and making the cells practicable have been rapidly advanced.
A typical structure of solid oxide type fuel cells is basically a stack obtained by stacking a large number of cells wherein an anodic electrode is formed on one face side of a planar solid electrolyte self-supporting film and a cathodic electrode is formed on the other face side. In order to make the power generation performance of the fuel cells high, it is effective to make the solid electrolyte self-supporting film dense and thin. This is based on the following reason. The solid electrolyte self-supporting film needs to have denseness for blocking the mixing of a fuel gas which is a power generation source with air surely, and an excellent ionic conductivity capable of suppressing electric conductance loss as much as possible. For this purpose, the film is required to be as thin and dense as possible. Moreover, a large stacking-load is imposed on the solid electrolyte self-supporting film since a fuel cell has a structure wherein a cell having an anodic electrode, a solid electrolyte self-supporting film and a cathodic electrode and a separator for separating and circulating a fuel gas and air are alternately stacked many times. Additionally, the operation temperature thereof is about 700 to 1000° C.; thus, the fuel cells receive considerable thermal stress. Accordingly, the fuel cells are required to have high-level strength and thermal stress resistance.
From the viewpoint of such required properties, a ceramic sheet made mainly of zirconia is mainly used as the material of the solid electrolyte self-supporting film for a solid oxide type fuel cell. A cell, wherein anodic and cathodic electrodes are formed on both faces of the sheet by screen printing or the like, is used.
The present inventors have been advancing research on such planar solid electrolyte self-supporting films for solid oxide type fuel cells for some time, and the research has been advanced so as to aim to make the thickness as small as possible for the purpose of decreasing ionic conductance loss while keeping physical properties and shape properties resisting stacking-load or thermal stress (preventing cracks based on local stress by decreasing undulations, projections, burrs and others) and, further, so as to aim to make the surface roughness appropriate for the purpose of heightening evenness and adhesion of the printed electrode. Previously, the present inventors suggested techniques disclosed in JP-A 2000-281438, JP-A 2001-89252, JP-A 2001-10866 and others.
These techniques made it possible that the solid electrolyte self-supporting film is largely thin and dense, and further the strength which resists stacking-load generated when cells are stacked, the thermal stress resistance, together with the adhesion and evenness of printed electrodes, are largely improved by improving the shape property, that is, decreasing undulations, projections, burrs and others.
Subsequently, the present inventors have been advancing research in order to improve the performance of fuel cells. This time, research has been made to aim to modify the property of electrode support substrates for support film type cells instead of the modification of the property of ceramic sheets used as solid electrolyte self-supporting films. This is based on the following reason. Ceramic solid electrolyte self-supporting films are more easily cracked by stacking-load as the films are made thinner; therefore, there is naturally generated a limitation in making the films thin and there is generated a limitation in decreasing in the ionic conductance loss.
In order to obtain cells having structure strength suitable for practical use in the case that thin solid electrolyte films are used therein, electrode support substrates are jointed, as supporting members for the cells, in between the cells or their electrodes are caused to have a sufficient thickness. The substrates have electrical conductivity for electric conduction. Furthermore, the substrates are made of porous ceramic material through which a fuel gas that becomes a power generation source, air, or exhaust gas (carbon dioxide, water vapor and others) generated by burning these gases can permeate and diffuse, which is different from the above-mentioned solid electrolyte self-supporting films.
In recent years, the following method has also been investigated. A method of forming an anodic electrode on a porous electrode support substrate by screen printing, forming a solid electrolyte film thereon by coating or the like, and further forming a cathodic electrode thereon by screen printing or the like to produce a cell, thereby making the solid electrolyte film still thinner so as to decrease electric conductance loss still more.
The most important theme when such a method is realized is that a cell has even and excellent gas permeability/diffusibility throughout its electrode support substrate. This is because this support substrate must be a porous substrate having pores sufficient for allowing a fuel gas and others to permeate and diffuse through the substrate. Further, the substrate is desired to have an even distribution state of the pores in such a manner that the gas can permeate and diffuse evenly through the whole of the substrate.
Another property desired for the electrode support substrate is that a superior printing adaptability is given to the surface thereof so that an electrode wherein the number of defects is as small as possible can be printed. As described above, the electrode support substrate is required to have an appropriate electrical conductivity. Further, the substrate must be a porous substrate having pores sufficient for allowing a fuel gas and others to permeate and diffuse through the substrate. Thus, numerous openings are present in the surface thereof. Therefore, in order to make superior electrode-printing possible in spite of the presence of such openings, it is indispensable to clarify surface properties peculiar to the porous electrode support substrate since the surface properties prescribed about the above-mentioned dense solid electrolyte film cannot be applied, as they are, to the porous electrode support substrate.
Still another property desired for the electrode support substrate is that the shape property of the support substrate itself is improved so that burrs, projections, undulations and others, which become stress-concentrated spots when they receive stacking-load or thermal shock, are made as small as possible. This is based on the following reason. As described above, the electrode support substrate is required to have an appropriate electrical conductivity. Further, the substrate must be a porous substrate having pores sufficient for allowing a fuel gas and others to permeate and diffuse through the substrate; thus, numerous openings are present in the surface thereof. Therefore, in order to restrain the support substrate, even admitting that the substrate is such a porous sheet, from being cracked or damaged by local stress concentration caused when it receives stacking-load, it is necessary to restrain the generation of burrs, which are formed at its internal and external circumferential edges at the time of punching, and projections or undulations, which may be formed inside the substrate, as much as possible. Furthermore, the electrode support substrate which is intended in the present invention must be a porous body through which a gas can permeate and diffuse. Therefore, the shape property effective for the printability of a dense sheet, such as a solid electrolyte film, and effective for the prevention of stress concentration thereon cannot be applied, as it is, to the electrode support substrate.
The present invention has been made, paying attention to a situation as described above. An object thereof is to provide an electrode support substrate to which electrode or a solid electrolyte film may be applied by screen printing. The substrate has the following characteristics. The entire surface of the substrate is stable against a fuel gas and others; the substrate has superior gas permeability/diffusibility. The substrate is able to form a printed electrode and a solid electrolyte film that are even and closely adhesive. The substrate has such a shape property that even if a plurality of the substrates are laminated into a cell stack and each of the substrates receives a large stacking-load, the substrate is not easily cracked or damaged by local stress concentration.